OK, so here's the deal. I make up a nuc or split. It takes 21 to 28 days to get a laying queen. By that time, all the adult bees there when the nuc was made is dead. The brood present when the split was made is now the only field force when the new queen starts laying. These bees will be dead by the time the new queens first brood starts emerging. They will be the nurse bee force now for the queen to lay more, and by this time the field force is reduced quite a lot, but from now on over all population will start increasing. It takes about 3 brood cycles (63 days) to get the hive population stablized and growing, from what I've seen here.
You're hive with the original queen was actively laying so no brood production down-time, and she had all the field force to keep bringing in stores. At the time you made the splits, she was full of field force with a greatly reduced brood population to need feeding, since brood frames went with the splits. So this hive has field force, less brood, nectar flow, so it is able to store up lots more honey than the other splits, which kept losing population for the first 60 days or so.